Degrigny, C. Cleaning CH Artefacts Overview. 2010
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Transcript of Degrigny, C. Cleaning CH Artefacts Overview. 2010
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Cleaning of CH artefactsAn overview
Christian Degrigny
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End of 19th c.
F. Rathgen
- Berlin: cleaning ofencrusted coin treasuresby electrochemicalreduction
A bit of history
1950s
Plenderleith (UK)
- use of constantcurrent I to clean &stabilise iron & copperartefacts- otherwise cleaning byelectrochemicalreduction (Zn/NaOH)
XXX
XXX
-
+VI
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1970s
Hamilton (USA)
Techniques applied to marine
metal artefacts
1980s
Pearson andMacLeod (Australia)
Use of constant V (voltage)for the treatment of marinemetal artefacts
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Since 1990s
-better control of the reactions takingplace on the metal surface (systematic useof reference electrodes when establishing
electrochemical parameters);
- more research on conservation
protocols and determination of treatmentparameters;
- development of new approaches and
design of new tools.
Corrosion scientists
Conservators
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- mechanical (superficial damage:
tarnished silver, marine cast iron, leadartefacts)
- heat treatment (modification of themicrostructure due to overheating)
Often applied in combinationwith other treatments
An alternative to othertreatments?
ArcAntique
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Ecorr (corrosion potential)
ArtefactM
M Mn+ + ne-: oxidation
2H2O + O2d + 4e- 4OH-: reduction
All reactionstaking place at
the metal
surfacewithout external
current
+ -
Aqueoussolution
Choice of the electrolyte:behaviour of metals in solution
EVTEK
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V
Artefact
Referenceelectrode
Ecorr/SHE= V + Eref/SHE
Ag/AgCl: 0.2V/SHE
Calomel: 0.242V/SHE
Ref
Ecorr measurement
V
Artefact
Local measurement
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Monitoring of Ecorr with time in acertain solution
Ecorr
time
Passivation
Corrosion
No reaction
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Influence of the pH of the solution
Several results fromPourbaix diagrams:
-Prediction of possiblereactions within the field ofstability of the aqueoussolution (normally no cleaning);-Choice of the appropriateelectrolyte (pH) to favour thepassivation effect;
Pourbaix diagrams arecalculated from
thermodynamics
Fe
Field of stability of
the aqueoussolution
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7 14E/SHE
0
1
-1
7 14E/SHE
0
1
-1
7 14E/SHE
0
1
-1
7 14E/SHE
0
1
-1Au
pH
Cu
pH
Fe
pH
Al
pH
I
C
I
CCP
I
CC P
I
C P
From gold to aluminium
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Modification of electrochemical parameterswith external current
Potentiostat
Power supply
Battery
Electrolytic cleaning
ArcAntique
MAH, Genve
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Definition of experimental conditions:voltammetric curves
Scanning of potentials in the cathodicand anodic regions
Determination of treatments parameters Follow up of treatments (I=f(t))
X
XX
X
X
X
XX
X
X
CE
Cathode
RE
WEPC
Potentiostat
Pourbaix diagrams cannot be used
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Cathodic scanning: reduction reactions
Ecorr
Mn+ + ne- M
Ecp
2H2O + O2 + 4e- 4OH-
Oxygen dissolved
I (A)
E (V)
EH2
2H2O + 2e- 2OH- + H2
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Anodic scanning: oxidation reactions
I (A)
E (V)Eap EO2
MMn+ + ne-
2H2O O2 + 4H+ + 4e-
Ecorr
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Coupons to be used for the definitionof parameters ?
Real artefacts
?
ArcAntique
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Use of artificial coupons
Silver tarnish
Ecorr
I (A)
E (V)
Steel oxidised
ArcAntique
ArcAntique
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Practically use of a generator
X
X
XX
X
X
X
XX
X
+
_A
Application of specific potentials
(given in the literature) Measurement of thecorresponding I
but
Monitoring is difficult (with basicequipment)
V
Cathode
anodes
HECR Arc
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Art-Mtal
CS1: cleaning of tarnished silver
Saxophone in silveredbrass, Muse de laMusique, Paris
Daguerreotype
pH
E/SHE
0
-1
1 Ag+Ag2O3
Ag2O
7
AgAg2S
Ag
10
Sodium sesquicarbonate
?
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Ecorr
2H2O + O2 + 4e- 4OH
Ag
Ag2S
2H2O + 2e- 2OH- + H2
I (A)
E (V)
Ag
H2
Ag
Ag+ + e- Ag
Red. Ag
Determination of the parameters
Potential range
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Monitoring of tarnish removal
I (A)
t (min)
Cathodic reductionat EpmaxEpmax
1
2
34
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Art-Mtal
Saxophone insilvered brass, Musede la Musique, Paris
Art-Mtal
After mechanicalcleaning
In practice
(in 1% (w/v) sodium sesquicarbonate, pH=10)
Silver vase,private collection,CH
HECR Arc
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Problem of silver alloys Ag-Cu
Ag2SAg
Cu2OCu2S
Ag-Cu
Red. AgRed. Cu
I (A)
E (V)Ecorr
Heterogeneous surface after treatment
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MAH, Genve
MAH, Genve
Silver
Gilt silver
Cleaning of tarnished gilt silver
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2 steps 1. Reduction
(Ag on Ag and Au) 2. OxidationEcorr
Ag2SAg
Ag Ag+I (A)
E (V)
MAH, Genve
In buffered NaNO3 solution
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Ag
Ag
reduced
Ag
Au
Ag2S
Reduction of Ag2S and AgCl inbuffered NaNO3, vb=1mV/s
AgCl
1st step: cathodic polarisation
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Before
ArcAntique
During
ArcAntique
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Results on several plates
Validation of heterogeneitiesdetected from visual observation
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AgAg+
I (A)
E (V)
Epmax
2nd step: anodic polarisation
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when the metal cannot be immersed
Tarnished gilt silverstags (17th c.)
Definition of the parameters ofthe treatment with a potentiostat
Use of the electrolytic pen with apower supply to clean thetarnishing Collaboration with GCI
M
+ -
V
J. Paul Getty Museum
GCI
J. Paul Getty Museum
J. Paul Getty Museum
J. Paul Getty Museum
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CS2: Cleaning of marine objects
EVTEK
ArcAntique
ArcAntique
Provoking thebubbling of H2
KOH
Iron-based objects
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00,2
0,4
0,6
0,8
1
1,2
1,4
1,6
1,8
2
0 200 400 600 800 1000 1200 1400 1600
time (h)
0
0,5
1
1,5
2
2,5Voltage (V)
voltage
I
I
Intensity (A)
Monitoring of the voltage and current
At -1V/SHE in KOH solution
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+
DC powersupply
Stainless steel tank
Stainless steelmesh (anode)
Electrolyte (KOH1%)
Plastic board
Insulated copper wires
Mild steel rodcovered by aPVC tube
Plastic support
Device
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In practice
EDFArcAntique
ArcAntique-
Remainingmaterial
marine crust
cracks
Li iti th b bbli f H t th i i
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Limiting the bubbling of H2 to the minimum
ArcAntique
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lead objects
-1.3VPb2+ Pb
E (V/HSE)
I (mA)
Needed?
In KOH solutions
EDF-Valectra
EDF-Valectra
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copper-based objects
1
23
4
I
E
Natural
patinadestroyed
In sodium sesquicarbonate
EVTEK
ArcAntique
ArcAntique
Limitations: composite artefacts
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Limitations: composite artefacts
Rifles at their arrival
Documentation
Investigation:Waterlogged?
ArcAntique
ArcAntique
ArcAntique
A collection of rifles (17th c.),Mediterranean Sea
When mechanical cleaning is the only
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g yway
Pneumatic ultrasonic
ArcAntiqueArcAntique
ArcAntique
Storage in Hostacor + H O solution
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Pre-storage witha corrosion inhibitor
Storage in Hostacor + H2O solution
ArcAntique
t (hours)
Ecorr (V/HSE)
Hostacor 1%
Hostacor 1.5%
Hostacor 2%
300100
-0.6
-0.7
-0.8
Other applications of the electrolytic
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Porthole with wooden frame19th c. Finnish wreck
beforeEVTEK
EVTEK
pen
EDF-Valectra
To clean
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Electrolyte: mixture of triammonium
citrate. Voltage: 4,6 V and Current: 45 mA
after
EVTEK
EVTEK